The objectives of the proposed project are threefold: 1) to establish the spatial relationships of the domains in the Fc region of IgM; 2) to quantitate the dynamics of antibody function by measuring the segmental flexibility of IgM and the mobility of immunoglobulin receptors on B cells in the nanosecond and microsecond time range; and 3) to elucidate the tertiary structure of antibody combining sites and the contributions of the heavy and light chains. These problems will be investigated by using fluorescence and circular dichroism techniques. Topology will be determined by measurement of intramolecular distances between fluorophores at specific locations by means of resonance energy transfer. Flexibility and mobility will be determined from rotational relaxation times obtained by nanosecond fluorescence depolarization measurements. Tertiary structure of combining sites will be determined from the induced circular dichroism of bound hapten and will be correlated with idiotypic determinants and binding affinities. These techniques have been previously utilized in our laboratory and shown to be feasible.